JPH08236288A - Back light device - Google Patents

Abstract

PURPOSE: To provide a high efficient back light device with the efficiency lowering of a piezoelectric transformer reduced, by providing an A.C. high voltage generation part in the vicinity of the high voltage side terminal of a discharge tube. CONSTITUTION: A discharge tube 20 is provided on the upper part long-side side of a liquid crystal display part 40 and an A.C. high voltage generation part 10 is provided on the short side part on the side of a discharge tube high voltage side terminal 22 respectively. Since the part 10 and the terminal 22 near with each other in such an arrangement, a high voltage side wiring 32 is shortened to lessen suspended capacitance Cs to reduce the efficiency lowering of a piezoelectric transformer, thereby obtaining a high efficient back light device.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a backlight device used for a liquid crystal display.

[0002]

2. Description of the Related Art Generally, in a liquid crystal display, since the liquid crystal itself does not emit light, there is used a backlight device in which a discharge tube such as a cold cathode tube is arranged on the back or side of the liquid crystal display to emit light. .

In order to emit light from the discharge tube, an AC high voltage of several hundreds of volts or more is required although it depends on the length and diameter of the discharge tube itself. The AC high voltage generating section for generating this AC high voltage is shown in FIG. A high frequency inverter using an electromagnetic transformer as shown in is generally used. The operation of this electromagnetic transformer is such that the high-voltage output voltage V ₂ generated in the secondary winding 75 of the electromagnetic transformer 70 is connected to the discharge tube 20 such as a cold cathode tube through a ballast capacitor 81 that limits the tube current I _O. To discharge the discharge tube 20. Since this electromagnetic transformer has to be wound with a large number of thin windings, there is a limit to downsizing and cost reduction of the AC high voltage generator.

As another method, an AC high voltage generating circuit using a piezoelectric transformer is disclosed in Japanese Examined Patent Publication No. 55-26600, Japanese Unexamined Patent Publication No. 52-13578, and Japanese Unexamined Patent Publication No. 5-11.
It is shown in Japanese Patent No. 4492. Piezoelectric transformers are much simpler in structure than electromagnetic transformers, so they can be made smaller, thinner, and cheaper. The principle and characteristics of this piezoelectric transformer are shown in "Characteristics of Piezoelectric Transformer and Its Application" in the July 1971 issue of the electronic magazine "Electronic Ceramics" published by Gakudensha.

FIG. 6 shows a Rosen type piezoelectric transformer announced by CA Rosen of the United States in 1956. The principle of this Rosen type piezoelectric transformer will be described with reference to FIG. 6. Reference numeral 2 is a plate-shaped piezoelectric ceramic element made of, for example, lead zirconate titanate (PZT). A pair of input electrodes 4 and 5 provided by, for example, silver baking is formed on the lower surface, and an output electrode 6 is formed on the right end surface by the same method. The left half drive section of the ceramic element 2 is preliminarily polarized in the thickness direction, and the right half power generation section is preliminarily polarized in the longitudinal direction in the directions indicated by the arrows.

In the piezoelectric transformer thus formed, when an AC voltage having a frequency substantially the same as the natural resonance frequency in the length direction of the ceramic element 2 is applied from the AC voltage source 8 between the input electrodes 4 and 5, the ceramic element 2 Causes strong mechanical vibrations in the longitudinal direction, which causes electric charges to be generated by the piezoelectric effect in the power generation section in the right half, and an output voltage V _O is generated between the output electrode 6 and one of the input electrodes, for example, the input electrode 5.

Output capacitance of piezoelectric transformer (output electrode 6
The capacitance between the input electrode 5 and the input electrode 5 is as small as 4 to 6 pF in a practical Rosen type piezoelectric transformer. Therefore, if electrostatic capacitance is added in parallel to the output of the piezoelectric transformer, its characteristics are greatly affected. FIG. 7 is a block diagram of a backlight device using a piezoelectric transformer.
In a portable information terminal device or a personal computer in which a liquid crystal display is used, the case is made of a conductive member in order to prevent electromagnetic noise interference, and a common potential (GN in the figure is used).
D), the high voltage side wiring 32 and GND
The electrostatic capacitance, which is called stray capacitance C _S , increases. Table 1 shows the measurement results of the efficiency of the piezoelectric transformer alone when the capacitance is added in parallel to the output of the piezoelectric transformer 18. The piezoelectric transformer 18 used for the measurement is a 6-layer laminated piezoelectric transformer, and the discharge tube 20 has a tube diameter of 3.0 mm and a tube length of 210 mm.
The results are for a tube current of 5 mA with a cold cathode tube of mm. Thus, it can be seen that the efficiency is significantly reduced when the capacitance is added to the output of the piezoelectric transformer 18.

[0008]

[Table 1]

[0009]

A high voltage output piezoelectric transformer has a small output electrostatic capacitance, which causes a reduction in efficiency due to the influence of stray capacitance. For this reason, the performance obtained by the AC high voltage generating unit alone changes greatly when mounted in the backlight device, and there is a case where the problem of efficiency decrease occurs.

The present invention focuses on the above problems,
It was created to solve this effectively. An object of the present invention is to provide a highly efficient backlight device.

[0011]

In order to solve the above problems, the present invention is characterized in that a piezoelectric transformer type AC high voltage generator is installed in the vicinity of a high voltage side terminal of a discharge tube. It is a device. Further, the piezoelectric transformer has a first region polarized in the longitudinal direction and a second region polarized in the thickness direction, and in the second region, piezoelectric bodies and internal electrodes are alternately laminated, It is preferable that the electrodes are connected to the external electrodes every other layer, and the two regions are manufactured by integral firing.

[0012]

Since the present invention is configured as described above, the stray capacitance generated by the wiring connecting the AC high voltage generating section and the discharge tube is small, so that the efficiency reduction of the piezoelectric transformer is small. Therefore, a highly efficient backlight device can be obtained.

[0013]

Embodiments of the present invention will be described below with reference to the drawings. First, a method of manufacturing the piezoelectric transformer used in this embodiment will be described in detail. A green sheet of PZT-based ceramics was produced by the doctor blade method, a platinum internal electrode (input internal electrode) was printed on a part of the green sheet by a screen printing method, and the sheets were laminated and pressure-bonded and baked. Tie. After that, cutting and polishing are performed, and an input external electrode and an output electrode are provided by silver baking, the internal electrode is connected to the external electrode, and the internal electrode is connected to the external electrode. At this time, in order to prevent contact between the internal electrode and the external electrode and between the internal electrode and the external electrode, an insulating layer is provided between these electrodes. The polarization is completed in the thickness direction of the drive unit and the longitudinal direction of the power generation unit. The following experiment was conducted using this piezoelectric transformer.

FIG. 1 is an explanatory view of a first embodiment of a backlight device according to the present invention. The discharge tube 20 is provided on the upper long side of the liquid crystal display 40, and the AC high voltage generating section 10 is provided on the short side (the right side in the figure) on the side of the discharge tube high voltage side terminal 22. With this arrangement, the AC high voltage generator 10 and the discharge tube high voltage side terminal 22 are close to each other, so the high voltage side wiring 32
Becomes shorter and the stray capacitance C _S becomes smaller. Therefore, the piezoelectric transformer efficiency is slightly reduced. FIG. 2 is an explanatory diagram of the second embodiment of the backlight device according to the present invention. Since the AC high voltage generator 10 is provided in the immediate vicinity of the discharge tube high voltage side terminal 22, the stray capacitance C _S is smaller than that in the first embodiment. For comparison with these examples, the discharge tube 20 is provided on the upper long side of the liquid crystal display 40 as shown in FIG. An example in which the AC high-voltage generator 10 is arranged in Comparative Example 1 is shown in FIG. 4, and the discharge tube 20 is provided on the upper long side of the liquid crystal display 40 as shown in FIG. Table 2 shows the results of measuring the loss of the AC high-voltage generating unit 10 using Comparative Example 2 in which the unit 10 is arranged. The piezoelectric transformer 18 of the AC high voltage generator 10 used for the measurement is a six-layer laminated piezoelectric transformer, and the discharge tube 20
Shows the result when the tube current is 5 mA with a cold cathode tube having a tube diameter of 3.0 mm and a tube length of 210 mm.

[0015]

[Table 2]

As described above, it was found that the example of the present invention can reduce the loss by 20% or more as compared with the comparative example.

[0017]

According to the present invention, since the stray capacitance of the piezoelectric transformer output becomes small, the backlight device is highly efficient.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of a first embodiment of a backlight device according to the present invention.

FIG. 2 is an explanatory diagram of a second embodiment of the backlight device according to the present invention.

FIG. 3 is an explanatory diagram of a backlight device using a conventional piezoelectric transformer.

FIG. 4 is an explanatory diagram of another backlight device using a conventional piezoelectric transformer.

FIG. 5 is a circuit diagram of an AC high voltage generator using a conventional electromagnetic transformer.

FIG. 6 is an explanatory diagram showing the operating principle of the Rosen type piezoelectric transformer.

FIG. 7 is a configuration diagram of a backlight device using a piezoelectric transformer.

[Explanation of symbols]

2 piezoelectric ceramic elements, 4 input electrodes, 5 input electrodes, 6 output electrodes, 10 AC high voltage generating section, 18 piezoelectric transformer, 20 discharge tube, 22 discharge tube high voltage side terminal, 24 discharge tube low voltage side terminal, 30 back Light device, 40 liquid crystal display.

Claims (3)

[Claims] 1. A backlight device comprising an AC high voltage generator installed near a high voltage side terminal of a discharge tube. 2. The backlight device according to claim 1, wherein a piezoelectric transformer is used in the AC high voltage generator. 3. The piezoelectric transformer has a first region polarized in the longitudinal direction and a second region polarized in the thickness direction, and piezoelectric substances and internal electrodes are alternately laminated in the second region. The backlight device according to claim 2, wherein the internal electrodes are connected to the external electrodes every other layer, and the two regions are manufactured by integral firing.